great project!
my 2 cents - before you do any pouring, make sure you do a test pour. That means create a small box using the same plexiglass material and pour some envirotex in it. That way, you can be a 100% sure on how the materials will react.
As for the surface, if you are not able to sculpt the envirotex, you can always get some acrylic gel to form the waves after the envirotex has cured. The trick to using acrylic gel is not to apply it too thick because it may not dry perfectly clear, again - it's important to test.
Here's a river diorama I did, but instead of envirotex, I used gedeo crystal resin (epoxy based resin), which I guess is similar to envirotex. However for this, I sculpted the base and waves in styrofoam, made a silicon rubber mould, cast the water in resin, and applied acrylic gel as a final texture. I had to use a sander to polish the sides and coated it with clear varnish (sanded and polished). A lot of work, whew. If I had to do it over again, I'll probably try a different approach.
https://armorama.kitmaker.net/forums/197487&page=1